How can someone take biro and produce this when I struggle to legibly write my own name

Robin Hobb did a series of books called the rainwild chronicles. The leaders of different cities used to send letters to each other and they were included at the start of each chapter.

The people in charge of ‘posting’ the letters used to add messages to each other to the letters. that side story, the way it was done was the best thing I have ever read.